A May 7, 2012, AP story broke the news that the CIA had thwarted an al Qaeda plot to blow up a U.S.-bound jetliner around the anniversary of the killing of Osama bin Laden by American commandos. The story, which included reporting by five staffers, said the plot was significant in part because the White House had told the public that it had no information about planned attacks around the anniversary.

It came two-and-a-half years after a similar attempt by al Qaeda operative Umar Farouk AbdulMutallab, who concealed a bomb in his underwear and unsuccessfully attempted to detonate it on a flight to Detroit. The 2012 attempt involved a more sophisticated bomb, the AP reported.

"The AP learned about the thwarted plot last week but agreed to White House and CIA requests not to publish it immediately because the sensitive intelligence operation was still under way," the news agency report stated. "Once officials said those concerns were allayed, the AP decided to disclose the plot Monday despite requests from the Obama administration to wait for an official announcement Tuesday."

Sources later told CNN that the operative who was supposed to have carried the bomb had been inserted into al Qaeda's Yemeni affiliate by Saudi intelligence, and that the device had been handed over to U.S. analysts. One source said Saudi counterterrorism officials were upset that details of the operation had emerged in the United States because they had a network of agents inside the Yemeni branch who could have been compromised by leaks from Washington.

Without confirming the focus of the probe, Attorney General Eric Holder said Tuesday that the disclosure was "within the top two or three most serious leaks I've ever seen."

"It put the American people at risk, and that is not hyperbole," he said.

Holder said he has recused himself from the investigation after being questioned by FBI agents as part of the probe, but he defended the Justice Department's decision to subpoena the AP records. AP President Gary Pruitt has called the subpoenas -- which the news agency learned of last week -- a "massive and unprecedented intrusion" into its reporting.

"Finally, they say this secrecy is important for national security," Pruitt wrote to Deputy Attorney General James Cole on Tuesday. "It is always difficult to respond to that, particularly since they still haven't told us specifically what they are investigating."